A transmission ratio variable mechanism capable of varying a transmission ratio as a ratio of an output rotation angle to an input rotation angle is known (for example, refer to Patent Documents 1 to 5).    Patent Document 1: Japanese Unexamined Patent Publication No. H05-105103    Patent Document 2: Japanese Unexamined Patent Publication No. 2004-256087    Patent Document 3: Japanese Unexamined Patent Publication No. 2005-162124    Patent Document 4: Japanese Unexamined Patent Publication No. 2005-67284    Patent Document 5: Japanese Unexamined Patent Publication No. 2007-145067
The transmission ratio variable mechanism includes, for example, as described in paragraph number [0015] of Patent Document 5 described above, a motor as a drive source and a harmonic drive gear as a differential mechanism. A rotation of a steering shaft accompanying a steering operation is superimposed on a rotation of the steering shaft based on the motor driving. Accordingly, the rotation of the steering shaft to be input into a rack and pinion mechanism is accelerated or decelerated so that a transmission ratio between a steering wheel and a turning wheel is altered.
In a motor vehicle steering system including this transmission ratio variable mechanism, it has been demanded to shorten an entire length of the system. An object of the present invention is to satisfy this demand.